Selective signaling circuit



3 Sheets-Sheet 1 Filed March 20, 1947 FOURTH BANK RC Fl6.l.

IOI.

TONE SIGNAL IOI r /m., Y 5 li ix N .B E 9 2 mw D 2 WW M DK3 0' B m w mm CA WA T m5 i [N A w J .b W 6 .MIM G .b 2 Y Uw .b b a W. B E I mg 8 M l 9 m 0 /fi b B m a 7 I 6 A 9 l O 3 Z 3 9 8 8 R w ,1? W 5L 6 MW (MW m 3 41 I T 9 m f? M W a R m 5 R w P .b I 9 l m 2/3 I m G F 2 MP; f M 1 m Dec. 28, 1948. HERBST 2,457,149

SELECTIVE SIGNALING CIRCUIT Filed March 20, 1947 3 Sheets-Sheet 2 FIG. 2

FIFTH BANK 1N VEN TOR. JOHN A. HERBST" Dec. 28, 1948. J. A. HERBST l M 3 smc'nvn SIGNALING CIRCUIT Filed March 20, 1947. 3 Sheets-Sheet 3 IN VEN TOR. JOHN A. HERBST BYW fl TTOZENEY Patented Dec. 28, 1948 SELECTIVE SIGNALING CIRCUIT John Amos Herbst, Pompton Lakes, N. J., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 20, 1947, Serial No.'735,939

11 Claims. 1

This invention relates to radio-telephone systems and particularly to equipment which is require'd for calling a selected station through a radio channel.

In certain respects the method of selective si naling which is herein described is similar to that which was disclosed in a co-pending application of David Talley, Ser. No. 716,137, filed December 13, 1946. In Talleys application it was stated that his transmitting apparatus was designed to avoid the use of call numbers containing the same figure in adjacent digits of the code, for example 2233 and 6775. The avoidance of such numbers was required in order that Talley's system might suitably cooperate with selective receiving systems of the type disclosed in two copending applications of Robert C. Ferrar and Gerald Menhennett, joint inventors, these applications being identified as Ser. No. 695,544 filed Sept. 7, 1946, and Ser. No. 697,670, filed Sept. 18, 1946.

According to my invention any desired sequence of frequency selective impulses may be used to compose a call signal. My system employs an ordinary telephone dial for transmitting trains of pulses corresponding to the digits of a fourdigit number. The same principles of operation are applicable to call numbers having more or less than four digits. Connected with the dial for pulsing purposes is a series of storage devices, preferably of the rotary switch type. These rotary switches are first stepped into position to store the ca l number. Immediately thereafter the call signal itself is transmitted over a radio channel. At this time different low frequency networks are chosen for controlling the modulation frequencies generated by an oscillator and then applied to modulate a sub-carrier wave which is superimposed upon a radio frequency carrier wave. In order to transmit the several audio-frequency signals which represent the digits of a cal number, relay means are employed which pick up the stored digits of the number successively. The operation is completely automatic after the dialing has been completed by the control terminal operator.

A fundamental feature of my invention is that call numbers containing the same figure in adjacent digits need not be avoided, because, in addition to ten selective frequencies which are 'used to identify the figures 0, 1, 9, I provide means for transmitting an eleventh. selective frequency which always denotes a repeated figure in the call number. Therefore, stations which are identified by call numbers such as 2234 2 can be made responsive to a sequence of cod pulses like this: I: in f3 f4, where 1 represents a pulse of any modulation frequency and its sub-' script represents a frequency corresponding to one of the ten figures or a repeat figure in the call number. Since the selector at the receiving station contains tuned reeds arranged and adapted to respond to different modulation frequencies of the code signal it may readily be seen that, whenever necessary, a reed tuned to in may be chosen to complement the chain of sequentially operable reeds for the several digits of a call number.

It is an object of my invention to provide a selective call signaling system which may be associated with any land line telephone system and with a radio telephone installation, and which will enable call numbers to be used containing any conceivable sequence of figures.

It is another object of my invention to provide call transmitting equipment which will select the frequency components of a call signal in accordance with the figures in the several places of digits of the number, except that an eleventh frequency component will be chosen to take the place of any immediately repeated figure in the code number.

Still another object is to provide comparison means operative betweendifferent sections of a storage device whereby the call number as set up therein may be translated into a succession of frequency-selective pulses representing any desired number, repeated figures being represented by an eleventh frequency.

Other objects and features of my invention will be made apparent in the detailed description to follow. This description is accompanied by drawings in which:

Figs. 1, 2 and 3 are to be viewed as showing a single circuit diagram relating to the call signal transmitting equipment. These figures should be placed side by side, with Fig. 1 on the left, Fig. 2 in the middle and Fig. 3 on the right, in order to readily trace the conductors which extend from one figure to another.

Fig. 4 shows schematically an arrangement of vibratory reeds to be used at a station to be called. These reeds are arranged and adapted to vibrate sequentially only when responding to a code signal the pulses of which are characterized by modulation frequencies to which the reeds are tuned.

Reference is first made to Fig. 4 for an explanation of how the code signals are responded to at a receiving station. The signal responsive unit is one which contains four vibratory reeds if there are four digits in the number. These reeds are mounted so as to be influenced by one common magnet IIII, this being an A. C. magnet the coil of which is connected across signal input terminals I III. The reeds are each made resonant to one of the tone frequencies used as distinguishing modulations 'for identifying the code number of each station. The reeds are here designated f2, in, f: and f4 merely to exemplify their chosen frequencies for response to the call number 2234.

Each of the reeds vibrates between contacts Iii-I09, but normally they stay in open circuit relation to these contacts until they are vibrated in response to the incoming frequency to which any one of them is resonant.

Contacts I08 are connected to one terminal of a direct current source III, the otherterminal of which is grounded. The reed f: is connected which is a capacitor II 3.

Reed is is also connected to contacts I01 which are associated with reed fn. Reed In also has groundconnections through a parallel connected resistor I I4 and capacitor 5.

Reed is is associated with contacts I08 which are connected to reed In. Parallel connected resistor I I6 and capacitor I I1 are both connected to the reed is, as well as contacts I09 which are associated with reed f4. This last mentioned reed is in circuit with the winding of a relay H8 on the other side of which is a ground connection. Relay III possesses a locking circuit which may be traced from ground through the D. C. source III and thence through circuit breaking contacts I20 and locking circuit contacts 9 to the coil of relay III. This relay may close one or more pairs of contacts such as I2I in circuit with terminals I22 for actuating any designated calling device, such as an audible signal, or it may apply power to a radio receiver in order to condition the same to receive a telephone call.

In the operation of the circuit arrangement shown in Fig. 4 the incoming code signal, when composed of a sequence of different pulses which are characterized by tone frequencies to which the reeds are tuned, causes these reeds to be vibrated successively. When reed f2 vibrates, it impresses a charge upon capacitor I I3 where it is stored until reed In vibrates. The second pulse causes reed in to vibrate and to transfer the charge from capacitor I I 3 to capacitor where it is again stored until reed f3 vibrates. The third pulse vibrates reed fa and causes the transfer of the char e in capacitor I I5 to capacitor I ll. Also the fourth pulse which vibrates reed f4 transfers the charge in capacitor II! to the relay H8, thereby operating this relay and causing the call signaling device to be sounded. Resistors II 2, H4 and I I6 are provided for the purpose of dissipating any residual charges on the capacitors which might be retained between calls. In combination with the capacitors they possess a time constant which is substantially longer than the signaling period.

It will be observed that the frequencies of the code number must correspond with the chosen frequencies to which the reeds are tuned. Any call number for a different station which operates on the same carrier frequency will be ineffective in transferring the capacitor charges from one to another so as to actuate the relay I I8. Therefore the response to a call is made exclusively in a selected station according to the tuning of its own reeds and the order in which the reeds are placed.

Now, where the code number is one which would call for two successive pulses representing the same figure, it is necessary that the second pulse should be different in frequency from the first pulse. For this reason the frequency m is used. This avoids the simultaneous vibration of two adjacent reeds which obviously would transfer a capacitor charge, say, in capacitor 3 directly over to capacitor Ill. Or if the second and third digits of the number were to be alike and if no eleventh frequency were to be used, then a charge on capacitor I I5 would be transferred directly to relay I I8 as soon as the fourth digit pulse was received. Such an arrangement would obviously cause confusion and would be likely to produce a response in more than one station.

If, perchance, the composition of the code number is one wherein two like figures are separated by a different figure or figures, then the subto ground through a resistor I I2 in parallel with stitution of the eleventh frequency is unnecessary. The numbers 2323 and 2342 are examples. Although two reeds would vibrate simultaneously and twice during reception of the code number, there would be no skip-transfer of any capacitor charge beyond the point permitted by. successive vibrations of the reeds. The sequence of operation of the different reeds therefore controls the successive transfer of charges from one to an adjacent capacitor. Therefore the eleventh frequency is to be used only in place of an immediately duplicated figure in the second, third, or fourth digits of the number, and not when the duplicate figure is separated from its antecedent by adifferent figure. Also, the call number 2222 will be translated into the code signal-f2 in 12 in in order to meet the requirements as above explained.

The difference between using a call system which is limited to code numbers having nonrepetitive figures and one which may use any number from 1 to 9,999 will be clearly understood from the following considerations. If we assign frequency responsive characteristics to four reeds of a call selector at ,the receiving station and designate these frequencies f0, f1, f9 and avoid the use of successive like frequency emissions, the total usable number of combinations is given by the expression P=n.(n--1) where P=number of combinations n=number of frequencies a.=number of digits (reeds) whereas if all combinations were used, the total number obviously is From the above the very considerable advantage of using an eleventh frequency in place of a repeated frequency for any of the pulses of the code signal will be appreciated. With the limitation that successive frequency emissions must not be the same, the total number of combinations possible is 7,290, whereas by substituting an eleventh frequency to transmit a repeated figure in the number, 9,999 numbers may be listed. One of the proposed uses for this system is for urban and highway mobile radio telephone service. The normal dialing service of the land line telephone system is, therefore, capable of extension to mobile telephone service by means of my improved system and without the need for avoiding duplicate figures in the call number code.

Referring now to Fig. 1 a conventional telephone dial is shown together with a rotary switch RC and various control relays the respective functransmitter 51 and a radio receiver;96 are diagrammatically indicated. A plurality of radio receivers having geographically spaced antennas may be included in a diversity receiving system if desired for wider coverage of the area in which mobile stations could be reached for communicatlon with a given control station.

In Fig. 2 I show a set of rotary switches TH, H, T and U, having stepping magnets II, I8, I8 and 20 respectively. Switch RC (Fig. 1) has four banks of arcuately disposedcontacts and associated wiper brushes, and the other rotary switches (Fig. 2) have five contact banks each. The RC switch is used to direct the dialing pulse trains into the other rotary switches successively for storing the four digits of a call number.

In Fig. 3 I show an oscillation generator 3| having a plurality of time-constant circuits H which are arranged for selective connection to the input electrodes of a discharge tube 3|. Relay means are also shown whereby successive selection of different time-constant circuits may be made, so that the generator will emit a chosen series of low frequency tone signals which represent the digits of a selective call signal.

The telephone dial I will be understood to comprise a pulse-producing circuit-breaker connected across conductors 2 and 3. The dialing circuit may be traced from ground through'contacts 6 of a dialing key I, relay 4 (which is preferably shunted by a resistor 5), the circuit-breaking contacts (not shown) of the dial I. contacts 8 of said key I, the coil of a quick-acting relay 9, and thence through a direct current source I I to ground. A resistor 82 and a capacitor 83 (in series therewith) are preferably connected between conductors 2 and 3 for spark suppression at the contacts of the dial I.

A pilot lamp P is lighted by power from a source I Ia upon closure of contacts a and b on.

relay 4, the lamp serving to indicate that the system is in readiness for dialing the code of the called mobile station. Lamp P remains lit until key I is restored to normal. Relay 4 is of the slow-release type, as indicated by the symbol SR within its outline. Other slow-release relays are similarly identified by this symbol.

For the sake of brevity in tracing the circuits, relay contacts will hereafter be referred to by a letter or letters having a numerical prefix corresponding to the reference for the contact-controlling relay itself. Two letters separated by a hyphen indicate closed relay contacts.-

Relays 4 and 9, being in series, are energized immediately upon operating key 1. "Contacts 9a and 9b are closed for the purpose of energizing a slow-release relay l2, the circuit for which may be traced from source I I through the coil of relay I2, through relay contacts 9a-b and thence through normal contact I3 on the first bank of a rotary switch RC, to the grounded brush BI. At the outset this and all of the other rotary switches shown in Fig. 2 stand with their brushes set on their normal contacts.

When relay I2 pulls up, its circuit is locked, as can be seen by tracing it from source II through its own coil, contacts 9a-'b and contacts I2a-b to ground. Relay I2 remains operated during the dialing of the complete code number, and is released by restoring key I to normal. The closing of relay contacts I2cd prepares a circuit through the coil of a slow-release relay I4 and thence to the source II by which it is energized. This circuit is completed through contacts 9c-b and brush BI on the first and subsequent breaks of the contacts in the dial I.

Relay I4 operates to close its contacts a and b, also its contacts 0 and d. Contacts I4ab complete a circuit through the stepping magnet II for rotary switch RC. Circuit interruptions produced by the dial I are used to release relay I momentarily. So, while slow-release relays I2 and I4 remain operated each release of relay 3 closes an operating circuit for relay I3, this circuit being traced from source I I through the coil of relay I6, through relay contacts I4cd, relay contacts 9cb, relay contacts I2ab, and thence to ground.

Relay I8 counts the dialing pulses for each digital train. Closure of contacts IIiab causes brush B3 on the third bank of rotary switch RC to be grounded. This brush, therefore, directs the dialing pulses of successive digital trains into stepping magnets I1, I8, I9 and 20 of the rotary switches TH, H, T and U respectively, for setting the same to store the code number of the called station.

At the end of the pulse train for the thousands digit relay 9 remains operated which causes relay I4 to release after a short interval. The release of this relay de-energizes the stepping magnet I5 and causes the brushes of switch RC to move from position N to position #I. The next release and operation of the relay 9 for the second dialed digit will in a similar manner operate stepping magnet I8 for rotary switch H, thus storing the digit in the hundreds place. The rotary switches operated by stepping magnets I9 and 20 will be caused to store the remaining digits of the dialed code number. I

The rotary switch RC, upon making its fourth step, that is,-after the units digit has been stored. places ground potential upon a conductor 2| which leads to the coil of a relay 22 and thence to the positive terminal of a source 23, the nega I tive terminal of which is grounded. This operation, therefore, energizes relay 22 which looks up through its own contacts a and b until such time as the locking circuit is broken by the energization of a relay 24.

Relay 22 closes its contacts 0 and d for carrying a 300 volt positive potential through a resistor 25 and thence to a conductor 26 which is connected jointly to front contacts a of four relays, 21, 28. 29 and 30 respectively. These relays are used for successive transmission of the tone frequencies which are selected by the storage means of the rotary switches so that the digits of the call number maybe transmitted in proper succession.

In order to characterize the digits of the code number of different predetermined frequencies, an oscillation generator is used in association with selectable networks which are tuned to desired selective frequencies. The oscillator per se comprises an electron discharge tube 3I which is preferably of the pentode type. Its cathode is connected to ground through a cathode resistor 32 shunted by a capacitor 33. The anode is connected to a source of D. C. potential, preferably I The suppressor grid, is connected through a resistor II to an intermediate point t on the cathode resistor 32, this point being adjustable as in the case of a potentiometer. The suppressor grid is also connected to contacts e of relays 21, 28, and in order that different ones of the rotary switches may be used to select the proper time constant networks for association with the oscillator in transmitting the elements of the code signal.

There are ten time constant networks 4| and the like, each having its resistive and capacitive components diflerently adjusted for determination of different audio-frequencies which are to be generated. An eleventh time constant network I is also provided for selective use when the frequency in is to be transmitted. Each of these networks comprises two sections, one containing a fixed resistor '42, in series with an adjustable resistor 43, and a capacitor 44, while the other section contains a fixed resistor 45 in series with an adjustable resistor 45. The junction between the two sections is coupled to ground through a capacitor 41.

Any one of ten networks 4| and the like may normally be selected to be connected to the common part ofthe oscillator circuit, the connections being made through contacts and brushes in the third and fourth banks of a rotary switch, as

- determined by the brush positions and by the selected operation of relays 21, 28, 29 and 30. These connections are, of course, made successively for generating the digit-representing frequencies for the thousand, hundred, ten and unit orders of the call number. During the closure of contacts (1 and e on one of the relays 21, 28, 29 and 30, therefore, the suppressor grid in tube 3| is connected through a brush and contact in the third bank of a rotary switch to the junction between capacitors 44 and 41 of the selected network. Also, the closure of contacts I and g on one of the relays 21, 28, 29 and 30 connects the screen grid of tube 3| through a brush and contact in the fourth bank of a rotary switch to the left hand end of the selected network 4| or the like.

In the preceding paragraph the selection of one of the ten networks 4| and the like was referred to as normal. The selection of network 4| representing the eleventh frequency will be understood to be necessary whenever successive figures in the code number are the same. The means for making this substitution will presently be described.

To those skilled in the art it will be understood that the circuit arrangement of tube 3| includes a selected one of the frequency-determining networks 4| and the like, or network MI, and a given selection causes the generation bf a tone frequency which may be utilized to characterize the signal for any digit of a code number. The oscillations are sustained by virtue of the feedback potentials which are derived from the tap t on the cathode resistor 32 and carried to the screen grid and the suppressor grid after suitable phase displacement by the two sections of the time constant network. The magnitude of the feedback potential is governed by the setting of the tap t along the cathode resistor 32, it being noted that this setting influences the self-biasing action with respect to the control grid, where the latter is connected to ground through the bias resistor 36.

The output from tube 3| is utilized in an amplifier tube 48, the control grid of which is cou- 8 pied across acapacitor tube 8|. Theamplifler tube 48, although shown as a triode, may, if preferred, be of a multi-grid type. As shown, however, the tube 48 possesses a cathode which is connected to ground through a cathode resistor 50. Itscontrol grid is connected to ground through a resist r 5|. Its anode is connected to the positive side of the 300 volt' source throughthe primary coil of a transformer 52. A by-pass capacitor 53 is connected across the terminals of the 300 volt source close to oneterminal of the transformer primary in order to by-pass parasitic frequencies. The secondary winding of transformer 52 has its terminals connected through conductors 54 to front contacts a and b of a slow release relay 55, the energization of which takes place as soon as rotary switch RC has moved away from its normal position N. The output from the transformer 52 may thus be delivered through a transformer 56 to a radio transmitter 51 from which the call signal is radiated.

In order to actuate the relays 21, 28, 29 and 30 successively and thereby to transmit the stored digits commencing with the thousands digit, relay 22 first impresses a charge upon a capacitor 58 which is in circuit with the coil of relay 21, upon being connected between the 300 volt source and ground, as when contacts '0 and d of relay 22 are closed. Relay 21 therefore operates for a 'drawn from the 300 volt source through relay I tacts of each of the relays 21, 28, 29 and 30 are effective in closing circuits through the brushes on the third and fourth banks of the rotary storage switches, it will be seen that different time constant networks become associated with the oscillator 3| for causing the selected elements of the code signal to be translated into frequency components which are then transmitted by the radio transmitter 51.

Relay 55 is operated when the rotary switch RC has made its first step to position #I. The operating circuit for relay 55 may be traced from ground through source 61, through the relay coil, through the interconnected terminals 2, 3 0 in the second bank of rotary switch RC and to its grounded brush B2. The operation of relay 55 connects the output of the low frequency tone oscillator circuit to the transmitter 51, as before described. A second function of relay 55 is to close a circuit from ground through a resistor 58 and thence through contacts 55g-h and over one conductor of the telephone line to the transmitter, whereat certain relays are actuated for closing the filament and anode circuits of a 7 kc. oscillator to be used as a sub-carrier in transmitting the call number. This sub-carrier is modulated by the low frequency tones to which the tuned reeds respond selectively at the called mobile station.

Other details of a suitable remote control sys- 49 to the screen grid in tem for the radio transmitter are not herein given as they are not essential to the inventive concept of this case. In general, they are in accord with usual radio telephone practice. Furthermore, it will be understood that no attempt has been made to describe or show completely such details of a practical radio-telephone system and switchboard equipment as are immaterial to a disclosure of the invention itself.

The circuit requirements for selection of the eleventh frequency network I4I will now be explained. The occurrence of the same figure in successive digits of the number is detected by means of a comparator system which includes the second contact banks in rotary switches TH and H, the first contact banks in rotary switches H and T, and the second contact banks in rotary switches T and U. Corresponding terminals (except in the normal position) in the second banks of rotary switches TH and H are interconnected. So likewise, corresponding terminals in the first banks of switches H and T are interconnected. Then again, corresponding terminals in the second banks of switches T and U are interconnected.

A transfer relay I30 is connected on one side to source 23 and on the other side to brush H2 in the second bank of rotary switch H. The circuit for this relay is completed whenever rotary switches TH and H are set to the same position other than the normal position. The complete circuit for relay I30 may be traced from a source 23, through the winding of relay I30, through brush H2, through one of the conductors interconnecting corresponding terminals of the second banks in these switches, through brush C2, and thence through contacts 22j-g to ground.

The section of the comparator which detects like figures in the 100s and 's digits of the number operates relay I3I whenever like figures occur in these digit places. The circuit for accomplishing this may be traced from the source 23 through relay I3I, through brush T1 through like contacts in the first banks of switches T and H, through brush H1 through contacts I30a-b and through contacts 22fg to ground. This relay will operate only when the same figure occurs in the 100s and 10s digits but with the exception that it will not operate if relay I30 has previously been energized by detection of like figures in the 1000s and 100's digits. The reason for avoiding operation of relay I3I in this case is that when a figure is twice repeated, the second repetition, as in the 10s digits, does not require the use of the eleventh frequency.

Relay I32 is operated upon detection of the same figure in the 10s and units digits of the number. The circuit for relay I32 may be traced from source 23 through its own coil, through brush U2 and a selected contact in the second bank of rotary switch U, thence through the corresponding contact in the second bank of rotary switch T, through brush T2 through contacts I3Iab, through contacts 22fg, and thence to ground. This circuit, like that of relay I3I, is subject to opening whenever the same figure is twice repeated, as in the number 2333. In this case the eleventh frequency is wanted only to represent the tens digit, since the network 4| which is tuned to frequency is will be selected for transmission of both the 100s digit and the units digit. Each of the relays I30, I3I, and I32 has movable contacts d and g which cooperate with back contacts e and h respectively and with front contacts 0 and f respectively.

Whenever one of these relays remains unenergized, the circuit connections from contacts of the sequence controlling relays 21, 28, 29 and 30 are completed through brushes and contacts of the third and fourth banks of rotary switches TH, H, T and U and through selected networks H. on the other hand, whenever one of the relays I30, I3l and I32 is operated, due to the repetition of a figure in the code number, the substitution of the eleventh frequency network I is made. In this case movable contacts d and g in one of these relays will close against their associated front contacts 0 and f respectively and complete the necessary frequency selective circuit through network I.

It will be noted that relays I30, I3I and I32 are timed to operate only during the operation of relay 22, since their operating circuits are returned to ground through relay contacts 22f-g. Upon the release of rezay 22 their coils will be de-energized and the relays restored to normal.

The holding times of relays 21, 28, 29 and 30 are concurrent with the transmission of the several digits of the call number, being governed by the times required to dissipate the charges in capacitors 58, 59, 60 and GI respectively. When relay 30 releases a discharge circuit is closed for capacitor 62. This discharge circuit includes the coil of relay 24 which is energized and performs several functions as follows:

(a) Conductor 59 is grounded through contacts 24e--f.

(b) A locking circuit for relay 24 is established until relay 22 releases, this circuit being traced from the 300 volt source through contacts 22e--d, contacts 240-11, the coil of relay 24 and thence to ground.

(0) One of two locking circuits for relay 22 is opened by pulling up contact 24a away from contact 24b. This leaves relay 22 subject to release by opening its second locking circuit through conductor 2|, the interconnected contacts 4, 5, 6 0 and the grounded brush BI in the first bank of rotary switch R0.

The above mentioned function (a) of relay 24 is for the purpose of restoring rotary switch BC to normal. The restoring circuit may be traced from. ground through contacts 24ef, conductor 69, through the interconnected terminals #I to #0 inclusive in the fourth bank of rotary switch RC and through the interrupter contacts 10 associated with the stepping magnet I5, and through the coil of magnet I5 to the source of I ll) by which it is operated.

Rotary switches TH, H, T and U will in turn be restored to their normal positions. In the case of the rotary switch TH the interrupter circuit through its stepping magnet I! may be traced from the grounded brush B2 on the second' bank of rotary switch RC and thence through terminals #I to #0 on the fifth bank of the rotary switch TH, and from the brush C5 thereof to the interrupter contacts II operated by the stepping magnet II.

In like manner the rotary switches H, T and U will be restored to normal, each through a grounded brush on the first bank of a preceding rotary switch, except that. if desired, the rotary switches H and U may be simultaneously restored to normal, this being preferably accomplished by joining their interrupter circuits together; the return of brush C1 on the first bank of rotary switch TH to its normal contact N, therefore, simultaneously restores switches H and U. Rotary switch T is preferably restored to normal( accord ing to the circuit as shown) after rotary switch U has returned its brush U1 to terminal N in the first bank. The restoring circuit may be traced from grounded brush U1, terminal N, interconnected terminals lll in the fifth bank of switch T, brush Ts, interrupter contacts operated by the stepping magnet i9, the coil of this magnet, and thence to the source lib.

Relays 22 and 55 are released when rotary switch RC has been brought to its homing position, since these relays up to that time were held by locking circuits through grounded brushes B1 and Ba respectively of rotary switch BC. The reLease of relay 22 will unlock relay 2. The release of relay 55 will reconnect the radio transmitter to the cord connections leading to the switchboard and to the operator's set.

The operators equipment for responding to toll calls and for dialing the call number of a remote radio station will be understood to be in accordance with standard telephone engineering practics and need not, therefore, be described in this specification. In order not to unduly complicate the specification and the drawings I have, therefore, shown merely a jack 93 to which an operator's set may be connected. Connection may be made through this jack to a switchboard and thence to a toll line or party line through which communication is to be established between a calling station and a called station. The jack 93 has its contacts connected respectively through voltage dividers 94 and 95 which are used for making connections both to the radio transmitter 5'! and to the radio receiver 95. In place of resistive units 94 and 95 hybrid transformers may be used if desired, in which case connections would be made between the jack 93, the radio receiver 96, and th radio transmitter 51 in accordance with conventional practice.

My invention is capable of modification of its structure in various ways without departing from its spirit and scope. The details of the embodiment herein described and shown are, therefore, not to be construed as imposing any limitations upon the breadth of the invention.

I claim:

1. In a selective call system means for storing a call number digit-by-digit, means for comparing adjacent digits one with another, means operative to translate the stored call number into a code signal of the type wherein different figures are represented by pulses each having one of ten different frequency characteristics, and means controlled, by said comparing means for introducing into said code signal a pulse having an eleventh frequency characteristic to represent the second of two adjacent digits that are numerically the same.

2. A call system according to claim 1 in combination with selectabLv responsive apparatus units individual to each of a plurality of receiving stations which are subject to call by said call system, said apparatus units including tuned reeds the resonant characteristics of which are suitably chosen to individualize the overall response to a given code signal.

3. In a selective call system, apparatus for transmitting a train of code signal pulses, each pulse having a distinctive tone frequency characteristic which is selected to represent the numerical value of a digit in a call number, said apparatus including means for storing said call number digit-by-digit, a comparator subject to control by said storage means and operable to determine instances wherein immediately successive digits of the stored call number are numerically the same, a tone frequency generator including therein a plurality of time constant circuits under selective control of said storage means jointly with said comparator, each time constant circuit being effective, when selected, to cause the generation of one of said distinctive tone frequencies, and sequence switching means for so composing said train of code signal pulses that normally one of ten tone frequencies is selected to correspond with the numerical value of each stored digit in said call number and an eleventh tone frequency is selected to represent the second of two adjacent digits that are numerically the same.

4. In a system for transmitting a series of signal frequencies which are chosen to represent the numerical values of successive digits in a call number, a pulse-producing dialing instrument, means responsive to pulses initiated by said instrument for storing a chosen call number digit-by-digit, an oscillator circuit arranged and adapted to generate diiferent ones of said signal frequencies according to the v numerical values of the digits in said call number, a comparator operable by said storing means for determining instances wherein immediately successive digits in the stored call number are numerically the same, and circuit means operable by said comparator whenever said instances are so determined for causing the frequency generated by said oscillator to be shifted to an eleventh frequency representative of said repeated numerical value.

5. The combination according to claim 4 and including relay means operable by said comparator for causing said oscillator to generate a normal numerical-value-representing frequency in the place of the third of three successive digits that are the same.

6. A station calling system of the type wherein each station has a distinctive call number to which it alone responds, said system comprising settable means for storing digit-by-digit the elements of a chosen call number, means for transmitting over a communications channel a series of signal frequency pulses wherein difl'erent frequencies are normally selected by said storing means to represent any of the ten possible numerical values of each digit, means for comparing immediately successive digits of a call number as stored in said settable means, and means operative under control of said comparing means to cause the substitution of an eleventh frequency in the place of one of said normally selected frequencies whenever such substitution is required to avoid repetition of the same frequency in successive pulses of said series.

'7. The combination according to claim .6 wherein the last said means comprises an eleventh frequency determining circuit supplementary to ten frequency determining circuits which are included in said means for transmitting normally selected signal frequency pulses.

8. In a system for transmitting a series of signal frequencies to which a given set of vibratory elements in a selected receiving station will respond in predetermined sequence, means for storing the call number of that station digit-bydigit, a modulation frequency generator having a plurality of alternatively operable frequency control networks arranged and adapted for selective connection thereto, relay means cooperative with said storing means to progressively make such connections between said generator and individual ones of said networks as will result in the generation of a train of modulation frequency pulses normally corresponding to the call number which is stored in said storing means, the frequencies of said pulses being chosen to obtain a resonant state in each of the vibratory elements of said selected station in said predetermined sequence, a comparator for detecting repetitions of a numerical value in successive digits of said call number, and means subject to control by said comparator and including a network having an eleventh frequency characteristic for representing a repeated numerical value, said means being operable to differentiate between the frequencies chosen to represent the first and second of said successive digits.

9. In a station selector system which utilizes storage means at the transmitter for controlling the selection of certain time constant networks to be successively connected in circuit with a low frequency generator, said storage means being arranged and adapted to 'store the numerical values of a call number digit-by-digit, the method of translating said call number into a selective calling code signal of the type to which a given set of vibratory elements in a selected receiving station will respond in predetermined sequence, said method comprising the steps of setting said storage means to positions which represent said numerical values of the call number, comparing said numerical values as between each digit and an immediately following digit to determine instances of like values, causing the frequencies produced by said generator as pulse elements of the call number to be normally determined by the closing of circuits through selected ones of said networks in accordance with the settings of digital parts of said storage means, and at times connecting a substitute network, which has an eleventh frequency characteristic, in circuit with said generator, thereby to differentiate between the frequencies of two immediately successive pulse elements whenever their numerical values are found by said comparison step to be the same.

10. Ina selective call system which utilizes tuned reeds at each station to be called and which comprises a transmitter arranged and adapted to send out a call signal composed of pulses which have different frequency characteristics capable of exciting selected sets of said reeds, the method of selecting suitable frequencies for each pulse of the call signal and of composing the pulse elements of said signal for sequential transmission which comprise, comparing successive digits of the call number of a wanted station to determine the presence or absence of repeated figures, selecting for each digital pulse of the call signal a frequency which is different from that of an immediately preceding pulse, fixing each frequency as one which in the absence of repetition is specifically assigned to a given one of the ten numerical values of a digit, assigning a fixed eleventh frequency to any pulse which represents the second of a twice-occurring figure, and causing the call signal so composed to be transmitted.

11. The method as defined in claim 10 and including the steps of receiving call signals simultaneously at a plurality of stations of thersystem, causing the reeds of different sets at respectively different stations to be excited in accordance with their resonance to the frequencies of the received call number pulses, and causing a selective call to be responded to at one-only of said stations whereat the arrangement of tuned reeds is suitably chosen for complete step-bystep response to the digital pulses of the transmitted call number.

JOHN AMOS HERBST.

No references cited, 

